Validating an email address in a form

A typo at sign-up means you will never reach that person, and every send produces a bounce.

The basic check

The format. That removes obvious errors but lets domain-name typos through.

The domain check

Does the domain exist and can it receive mail? That catches the classic typos in well-known providers.

What not to do

A confirmation field to retype. People copy and paste, and you then validate the same mistake twice.

The suggested correction

If somebody types a near-variant of a large provider, offer the correction rather than refusing. It is the most effective check and the least irritating.

Then confirm

The only check that genuinely proves an address works is a confirmation message with a link. Everything else is an estimate.